While fish oil contains omega-3 fatty acids that can have health benefits, taking too much can have side effects.

Fish oil is well known for its wealth of health-promoting properties. It contains two types of omega-3 fatty acids:

  • eicosapentaenoic acid (EPA)
  • docosahexaenoic acid (DHA)

Rich in these heart-healthy omega-3 fatty acids, fish oil supplements may relieve inflammation, reduce the risk of diabetes, and even relieve symptoms of inflammatory conditions like rheumatoid arthritis.

However, more fish oil is not always better, and taking too high a dose can potentially cause negative side effects.

This article will cover nine potential side effects of consuming too much fish oil or omega-3 fatty acids supplements.

Fish oil from fish vs. supplements

The Dietary Guidelines for Americans 2020–2025 recommend adults eat 8 ounces or more of a variety of seafood (fish or shellfish) weekly. They recommend slightly lower amounts for pregnant people and children.

Seafood contains beneficial omega-3 fatty acids. However, experts believe that other beneficial nutrients in seafood may contribute to the health benefits of consuming fish oil. Fish oil supplements do not contain these other nutrients.

It’s easier to exceed the recommended dosage of a vitamin or nutrient when you’re taking it in supplement form compared to when eating whole foods that contain it.

Learn more about taking vitamins and supplements.

Taking fish oil daily can help lower high blood pressure.

A 2022 meta-analysis of research suggests that a dose of between 2 and 3 grams (g) of omega-3 fatty acids may lower both systolic and diastolic blood pressure:

  • 2 g of omega-3 fatty acids: lowered systolic blood pressure by 2.61 and diastolic blood pressure by 1.64
  • 3 g of omega-3 fatty acids: lowered systolic blood pressure by 2.61 and diastolic blood pressure by 1.8
  • More than 3 g of omega-3 fatty acids: may have an increased effect

If you already have low blood pressure or you take medications to lower your blood pressure, taking fish oil supplements may make your blood pressure too low.

It’s best to talk with a doctor before taking fish oil or any new supplement, especially if you have other health conditions.

Some people experience acid reflux, or heartburn, and other gastrointestinal side effects after consuming fish oil. This may be due to its high fat content.

Research from 2019 that looked at side effects associated with fish oil noted that acid reflux and other digestive side effects were infrequent.

  • heartburn
  • belching
  • nausea
  • stomach discomfort
  • bloating

When and how you take fish oil may affect your symptoms. Taking a moderate dose and taking supplements with meals may help reduce acid reflux and relieve symptoms. Splitting your dose into a few smaller portions throughout the day may also help eliminate indigestion.

Diarrhea is a common side effect associated with taking fish oil and may be more prevalent while taking high doses.

According to the same 2019 research, people reported diarrhea and other digestive side effects with larger doses of fish oil that contained 3 to 4 g (or 3,000 to 4,000 milligrams) of EPA and DHA.

In addition to fish oil, other types of omega-3 supplements may also cause diarrhea.

If you experience diarrhea after taking omega-3 fatty acids, make sure you’re taking your supplements with meals and consider decreasing your dosage to see how it affects your symptoms.

According to the National Center for Complementary and Integrative Health (NCCIH), omega-3 supplements, like fish oil, can cause headaches.

However, research also suggests that fish oil supplements may reduce the severity of migraine episodes in people with migraine, a neurological disorder that causes severe headaches along with other symptoms.

Trimethylamine is a chemical compound in fish oil.

Though uncommon, some people may be unable to break down this compound in their bodies. This condition is called trimethylaminuria, or fish odor syndrome.

If you’re unable to break down this chemical compound, it can build up in the body and be released through:

  • breath
  • sweat
  • urine

Taking fish oil supplements if you have this condition can cause your bodily fluids to have a fishy odor. The smell can vary over time.

In people with cardiovascular disease and those at risk of cardiovascular disease, taking very high doses of fish oil supplements for several years may slightly increase the risk of atrial fibrillation (AFib).

AFib is a type of irregular heart rhythm or arrhythmia that interrupts the typical flow of blood through your heart. Having AFib can increase your risk of blood clots and stroke.

A large 2020 clinical trial noted this potential adverse outcome in people who took 4,000 milligrams (mg) of fish oil daily, which is much more than the amount provided in a single fish oil supplement.

One possible benefit of fish oil is reduced inflammation, which results from suppressing the body’s inflammatory responses.

However, the body uses inflammation as a tool to manage and treat infection, injury, and disease.

Suppressing these responses by taking high doses of fish oil daily (900 mg EPA plus 600 mg DHA) for more than several weeks may potentially reduce the body’s immune function.

Theoretically, fish oil may increase bleeding by reducing the speed at which blood clots. However, a 2018 clinical trial suggests that taking fish oil supplements did not increase bleeding during surgical procedures and decreased the number of blood transfusions.

A 2024 meta-analysis of clinical trials also suggests that omega-3 fatty acids are not associated with increased bleeding risk. However, the authors note that people taking high dose purified EPA may have an increased bleeding risk in theory, but that the risk wasn’t significant in practice.

Some doctors may still recommend you stop taking fish oil before having surgery. It’s also best to talk with your doctor before taking fish oil or other supplements if you’re on blood thinners like warfarin (Jantoven).

The body stores the extra vitamin A you consume. This means the levels present in your body can accumulate over time if you take in more than your body needs.

Certain types of omega-3 fatty acid supplements are high in vitamin A, which can be toxic if you consume too much of it. For example, just one tablespoon (14 g) of cod liver oil can provide 453% of your daily vitamin A needs in one serving.

Vitamin A toxicity can cause side effects such as:

  • dizziness and coordination problems
  • nausea
  • joint pain
  • skin irritation
  • severe headache

Long term, it could also lead to liver damage and even liver failure in severe cases.

It’s best to pay close attention to the vitamin A content of your omega-3 supplement and keep your dosage moderate.

The recommended dietary allowance of vitamin A for adults over 18 is:

  • males: 900 micrograms (mcg) retinol activity equivalents (RAE)
  • females: 700 mcg RAE
  • people who are pregnant: 770 mcg RAE
  • people who are breastfeeding: 1,300 mcg RAE

Intake recommendations for vitamin A are given in RAE to account for the different bioactivities of retinol and provitamin A carotenoids, which the body converts into retinol.

How much is too much?

The American Heart Association (AHA) and the Dietary Guidelines for Americans recommend 8 ounces or about 2 servings of fatty fish per week.

A typical supplement provides about 1,000 mg of fish oil, containing 180 mg EPA and 120 mg DHA, or about 300 mg combined. One teaspoon (5 milliliters) of liquid fish oil usually contains about 1,300 mg.

The Adequate Intake, or the amount needed for nutritional adequacy, can vary based on personal factors. These levels are:

  • Males age 14 and older: 1.6 g (1,600 mg)
  • Females age 14 and older: 1.1 g (1,100 mg)
  • People who are pregnant: 1.4 g (1,400 mg)
  • People who are lactating: 1.3 g (1,300 mg)

However, some studies discussed in an AHA review administered a higher amount, up to 4 g or 4,000 mg, for people with certain health conditions, such as heart disease or high triglyceride levels.

According to the European Food Safety Authority, omega-3 fatty acid supplements can be safely consumed at doses up to 5,000 mg daily.

As a general rule of thumb, if you experience any negative symptoms, simply decrease your intake or consider meeting your omega-3 fatty acid needs through food sources instead.

The bottom line

Omega-3 is an essential part of the diet, and supplements like fish oil have been associated with multiple health benefits.

However, consuming too much fish oil can potentially lead to side effects such as acid reflux and headaches.

Experts note that getting the omega-3s your body needs through eating seafood may have more benefits than taking supplements. It’s best to get the majority of the recommended dosage of omega-3 fatty acids from whole food sources.

But, if you cannot get the amount you need through seafood, taking supplements may help you reach an adequate level of omega-3 fatty acids.

Read about the different types of omega-3 supplements available and how to choose the right option for your needs.